This invention relates to a film material useful in the electrical industry which is fire or flame retardant. More particularly, the invention is directed to the use of a flame-retardant coating in conjunction with a backing film, and, in a preferred embodiment, a pressure sensitive adhesive layer.
Electrically insulative films, often in tape form, i.e., contained on a core, are utilized to wrap electrical components therewith. In many instances, the tapes contain a pressure sensitive adhesive thereon for obvious benefits. Such tapes typically must have characteristics such as insulation qualities relative to electrical shock, temperature stability, moisture and chemical resistance, toughness, etc. In recent years, it has become increasingly important to include flame retardancy as a desirable quality, such that any flame or fire produced by a shorting out of the electrical component enclosed by the tape, or by an outside source, will not be propagated by the tape itself.
In the past, various schemes have been proposed to provide fire retardancy to electrical tapes. In tapes containing a pressure sensitive adhesive, one common method is to include fire retardant components in the adhesive itself. One such system is disclosed in U.S. Pat. No. 4,061,826, wherein a halogenated compound is utilized in conjunction with a pressure sensitive adhesive to achieve flame retardancy. However, it has been ascertained that in such systems, the concentration of additives necessary to achieve satisfactory flame retardancy must either be so great that the basic adhesive properties are thereby degraded to an unacceptable level, or other functional deficiencies are noted. Similarly, in U.S. Pat. No. 3,515,578, there is disclosed the addition of a phosphate component to a pressure sensitive adhesive to provide fire retardance thereto, but again, the flame retardancy thereof is less than optimum, and the additive also has a softening effect on the adhesive itself, thereby rendering the characteristics of same somewhat less than optimal.
Furthermore, fire retardant adhesives do not prevent conventional thermoplastic backings from dripping when contacted by flame, which can cause a removal of the insulation from an electrical part, and most always allows for continuation of a fire.
Another approach has been to utilize intumescent coatings on backings to provide fire retardant tapes therewith, as is disclosed in U.S. Pat. No. 3,336,153. However, in many instances intumescent materials require excessive thicknesses to be effective, and are water sensitive, and therefore are inapplicable for electrical tapes.
Still another approach has been to provide fire retardant components in the film backing itself, with or without utilization of an adhesive system which is proposed to be flame retardant. However, this approach provides inadequate fire retardancy because the necessary high loading of fire retardant components makes processing of the backing difficult, if not impossible.
It has now been ascertained that by the utilization of a thermoset resin layer which is fire retardant, an electrically insulating film can be produced which contains excellent fire retardancy without reducing the electrical, physical and chemical properties of the film which are so necessary for functionality thereof. The resinous layer can be simply applied to a thermoplastic support film, and when a pressure sensitive adhesive is utilized, the resinous layer can be either interposed between the thermoplastic support film and the adhesive or can be applied to the backside of the support film. The resinous layer can be utilized with prior art electrical tapes which contain adhesives with fire retardant additives therein.